The invention relates to a method of isotope separation based on selective excitation of isotope species and more particularly to a method wherein the selective excitation is initiated by laser means.
For various nuclear applications it is exceedingly important that the fissile isotopes .sup.235 U and .sup.239 Pu be separated from or strongly enriched in admixtures with other isotopes of uranium and plutonium, respectively. Presently, the only economically viable method for producing uranium enriched in .sup.235 U is the gaseous diffusion process which requires very large capital investment and tremendous plant facilities. There is presently no practical scheme available for separating .sup.239 Pu from .sup.240 Pu, a separation which is highly desirable for certain military applications.
With the advent of lasers tunable to very narrow frequencies over a wide range of the spectrum, it has become apparent that by controlling the spectral response of the interaction of light with matter, it is possible, in principle, to produce selective reactions that can change the composition and properties of the matter. The conditions required to achieve such selectivity are: (1) high monochromaticity of the exciting light; (2) the selectivity of the primary process of light interaction with the matter (the existence of narrow nonoverlapping absorption lines); and (3) conservation of the induced selectivity in successive physical and chemical processes. See, e.g., R. V. Ambartzumian and V. S. Letokhov, "Selective Two-Step (STS) Photoionization of Atoms and Photodissociation of Molecules by Laser Radiation," 11 Applied Optics 354 (1972).
Laser art has advanced sufficiently that tunable lasers having bandwidths narrower than 0.0002 cm.sup.-1 are available so that the first condition is completely satisfied. Efficient amplification of narrowly tuned infrared oscillators can be accomplished with high pressure electron beam controlled electric discharge gas lasers. Such a scheme allows narrow bandwidth with high overall electrical efficiency. High overall efficiency can be obtained by use of parametric oscillator and phase matching techniques for tuning efficient visible and ultraviolet lasers such as lead atom, copper atom, and Xe.sub.2 lasers. Tunable dye lasers have sufficiently narrow bandwidths in the range 3600 to 7200 .ANG., although their electrical efficiency is lower.
The second and third conditions present substantial problems. For example, in principle the second condition can be met by the interaction of precisely tuned laser light provided that there exist certain discrete electronic and vibrational transitions of matter in the gaseous phase. Even if discrete transitions exist, it is frequently difficult to ascertain in a gaseous species which transitions are appropriate for selective interaction with tuned laser light.
Once selective excitation has been made to occur, there are numerous processes by which the selectivity may be lost. A primary loss mechanism is collisional energy transfer between molecules. Thus if the third condition is to be achieved, it is highly desirable that the selectively excited species be transformed to a stable or metastable state. One means by which the selectivity can be stabilized is through photoionization or photodissociation of an excited species. A problem, however, is that photoionization or photodissociation may not themselves be selective.
It is known in the art that the stabilizing effect of either photoionization or photodissociation may be used advantageously if they are separated from the selective excitation step through use of photons or light quanta of differing energies h.nu..sub.1 and h.nu..sub.2 . Photons of energy h.nu..sub.1 excite a certain state of the discrete energy spectrum in a particular species, and photons of energy h.nu..sub.2 photoionize or photodissociate the excited species. The energies of the photons satisfy the following conditions: EQU h.nu..sub.1 +h.nu..sub.2 &gt;E.sub.i,E.sub.d EQU h.nu..sub.2 &lt;E.sub.i,E.sub.d
where E.sub.i is the photoionization energy of an atom or molecules from the ground state and E.sub.d is the photodissociation energy of a molecule from the ground state.
The art indicates that this two-step process, or two-photon process as it is also known, is applicable to the separation of isotopes. A prerequisite for such separation is the existence of a suitable isotope shift in the absorption spectra of the element or one of its compounds so that only one isotopic species is excited by the tuned light.
In U.S. Pat. No. 3,443,087, issued May 6, 1969, Robieux et al. reveal a process for ionizing selectively a gaseous compound of an isotope which is a part of a mixture of isotopes which comprises irradiating the mixture of isotopes with light of two different wavelengths in two steps, the first irradiation by light of one wavelength serving to selectively excite the molecules of one isotope and the second by light of another wavelength serving to ionize the excited molecules. The ionized molecules are then subjected to electric or magnetic fields or a combination thereof to deflect them away from the unionized isotopic compound.
Using a first irradiation of infrared light and a second irradiation with ultraviolet light, Robieux et al. indicate that .sup.235 UF.sub.6 and .sup.238 UF.sub.6 may be separated according to the process of their invention. The rationale behind their two-photon process is that finely tuned energy available from absorption in the infrared region of the spectrum will selectively excite one of the uranium isotopes, preferably the .sup.235 U, but is inadequate to excite the isotopic compound which is absorbing it sufficiently to produce ionization. Line breadths in the ultraviolet spectral region, where there is sufficient energy to produce ionization, are larger than at lower frequencies so that it is much more difficult to achieve the requisite selective absorption in this region of the spectrum. That is, although photoionization can readily be produced by ultraviolet light, it is not likely to be selective. Through use of the two-step absorption process, one isotopic species is selectively excited by the infrared and then a sufficient amount of energy is provided by the ultraviolet (which is absorbed by both species) to just drive the excited isotopic compound past the ionization threshold, whereas the isotopic compound that remained in the ground state during the infrared irradiation is not sufficiently excited by the ultraviolet to be ionized even though it absorbs to substantially the same degree.
Reasonably sharp isotope shifts have been identified for uranium and its compounds, but at either very high or very low temperatures. The very high temperatures have been necessary for elemental uranium. Unfortunately, even at 1600.degree. C. uranium has a vapor pressure of only 1 micron, which is much too low to obtain any reasonable light interaction with the vapor. Thus a substantially higher temperature is required, and an isotope separation process based on the use of elemental uranium as the feed material does not therefore appear practical. Cesium uranyl chloride (CsUO.sub.2 Cl.sub.4) and cesium uranyl nitrate (CsUO.sub.2 (NO.sub.3).sub.3) enriched in .sup.235 U have shown an isotopic shift of 1.62 cm.sup.-1 at 20 K. While the spectral lines are sharp at 20 K., they become broad at 77 K. and cannot be resolved at higher temperatures. At the low temperatures at which the lines are defined, however, these compounds exhibit essentially no vapor pressure.
Certain isotopic shifts in the infrared spectrum of UF.sub.6 at room temperature have been determined by measurements on separated samples of .sup.238 UF.sub.6 and .sup.235 UF.sub.6 . The 623 cm.sup.-1 .nu..sub.3 (F.sub.lu) band shows a measured shift of 0.55 cm.sup.-1. Measurements on the other infrared bands indicate a shift of 0.1 to 0.2 cm.sup.-1 for the .nu..sub.4 (F.sub.lu) vibration, the only other of the six vibrations which should show a nonzero isotope shift. These measured shifts are gross in nature, however, and no fine line spectra were resolved.
Although Robieux et al. in U.S. Pat. No. 3,443,087 state that a chemical reaction may be used to separate the isotopes, they give no example of what chemical reactions will suffice or how such chemical reactions might be brought about. They consequently make no claims with respect to chemical separation. In a recent report, R. C. Farrar, Jr. and D. F. Smith review the literature dealing with photochemical means for isotope separation, with particular emphasis on the separation of uranium isotopes. See "Photochemical Isotope Separation as Applied to Uranium," Union Carbide Oak Ridge Gaseous Diffusion Plant Report K-L-3054, Rev. 1 (Mar. 15, 1972). Although photochemical dissociation of UF.sub.6 would have advantages over photochemical reactions involving two molecular species, Farrar et al. do not devote any discussion to it.